This invention relates to entrained flow coal gasifiers and in particular to a method of operating such gasifiers through a load range.
The entrained flow gasifier is essentially comprised of two zones. The first or the combustion zone generates the heat required for the gasification process. This zone is operated at near stoichiometric conditions to obtain the maximum heat and also to melt the ash so that it may be removed in the form of slag. As applied to combustion and gasification, stoichiometric is known to relates to the theoretical amount of oxygen required to completely burn the material being combusted. Where air is used this relates to the similar requirement of air. Off-stoichiometric relates to any ratio greater or less than the stoichiometric ratio while substoichiometric relates to a lesser amount of oxygen than required for theoretical complete combustion. The operation of this zone is off stoichiometric only to the extent required to reduce temperature where the materials forming the combustion zone cannot tolerate stoichiometric temperature.
The combustion products from the combustor zone are then mixed with incoming pulverized coal. The incoming coal is devolatilized and the carbon particles combine with the combustion products to form a gas which is largely carbon monoxide. This is an endothermic reaction obtaining its heat from the gases leaving the combustor. The gasification process continues until the temperature is reduced to a level at which the gasification rate is too slow for practical operation. Any remaining coal particles in the form of char may be recycled to the combustor.
After a gasifier is designed for optimum conditions at the maximum rating the same gasifier must be operated at reduced ratings. In such an operation the coal and air to the combustor is reduced while still maintaining stoichiometric conditions therein. Similarly the coal supplied to the reductor section is reduced in accordance with the reduced gas flow.